This invention relates to phase transfer catalysis. More particularly, it relates to the preparation of aqueous solutions of hexasubstituted guanidinium salts useful as phase transfer catalysts, and to the use of such solutions in the preparation of polyetherimides and intermediates therefor.
The use of hexaalkylguanidinium salts as phase transfer catalysts in various reactions is known. Many of these reactions are characterized by the necessity of conducting them under anhydrous conditions. This makes it very difficult to introduce measured amounts of the hexasubstituted guanidinium salt into the reaction mixture, since such salts are solids and are only sparingly soluble in the organic solvents employed.
Hexasubstituted guanidinium salts may be prepared by a series of three reactions: the phosgenation of a secondary amine to produce a tetrasubstituted urea; further phosgenation of the tetrasubstituted urea to form a chloroformamidinium chloride, also known as a "Vilsmeier salt" and sometimes designated as such hereinafter; and finally, the reaction of the Vilsmeier salt with further secondary amine to produce the hexasubstituted guanidinium chloride. The third of these reactions must be conducted under anhydrous conditions, and the second is preferably anhydrous also.
It is frequently found that preparation of the Vilsmeier salt in good yield cannot be achieved unless expensive solvents such as acetonitrile are employed or a substantial excess of phosgene is used. This is true, for example, of the method described in Kantlehner et al., Liebigs Ann. Chem., 1984, 108-126, which afforded the hexasubstituted guanidinium salt in 94% yield but employed acetonitrile as the solvent. A method employing phosphorus oxychloride in toluene is described in Example 1 of U.S. Pat. No. 5,132,423; it afforded the product in 87% yield. A method employing phosgene and toluene, described in Barton et al., J Chem. Soc., Perkin Trans., 1982, 2085-2090, resulted in a relatively low yield (85%) even with the use of a substantial excess of phosgene (molar ratio 1.95:1). The use of a large excess of phosgene, in any event, is undesirable since phosgene is toxic and is difficult to contain during the reaction. Moreover, disposal of a large excess of phosgene generally requires a sparging operation including passage through a caustic scrubber, presenting a potential hazardous waste problem upon disposal.
The product of the above-described series of reactions is invariably the hexasubstituted guanidinium chloride. It has further been found that the chlorides are hygroscopic, making their use under anhydrous conditions complicated. Therefore, they are often converted to other salts such as the bromides. This necessitates an additional reaction step, which is also expensive and therefore undesirable.
The use of hexasubstituted guanidinium salts as phase transfer catalysts in the reaction of bisphenol salts with halo- or nitro-substituted phthalimides to form bisimides, which are conventional intermediates for polyetherimide production, is described in Example 4 of U.S. Pat. No. 5,081,298. Its use in the direct preparation of polyetherimides by the reaction of bisphenol salts with similarly substituted bis(phthalimido) compounds is disclosed in U.S. Pat. No. 5,229,482. In each case, the guanidinium salts were added in solid form, which, as previously mentioned, presents numerous problems in commercial operations. Anhydrous conditions are mandatory in these reactions, since the presence of even traces of water can drastically decrease yields.
Methods of preparing hexasubstituted guanidinium salts in high yield and using them in a convenient manner as phase transfer catalysts in commercial operations, therefore, continue to be desirable subjects of investigation.